1. Field of the Invention
The invention pertains to an x-ray examination apparatus for producing x-ray images and comprising means for noise reduction. The invention also relates to means for noise reduction suitable for use in an x-ray examination apparatus.,
2. Description of the Related Art
An x-ray examination apparatus of said kind is described in the U.S. Patent Specification U.S. Pat. No. 4,367,490.
In that document an x-ray examination apparatus is described which produces a time-sequence of x-ray images, i.e. it produces a sequence of successive x-ray images and converts said x-ray images into video signals having signal amplitudes in correspondence with brightness of picture elements (pixels) of the x-ray images. An x-ray examination apparatus known from the cited reference comprises means for noise reduction by recursively temporally averaging of pixel values. An important noise source in x-ray imaging is shot noise that is to be attributed to the quantised nature of the x-ray photons. By recursive temporally averaging images random noise can be suppressed, because uncorrelated information contained in an image is averaged out. However, when, in a medical x-ray examination motion is involved of e.g. the wall of a beating heart, or a catheter being moved by a physician, recursive temporally averaging leads to loss of image quality. In particular moving edges in the displayed images would have long trailers behind them. In an x-ray examination apparatus as described in the cited reference noise reduction is omitted for pixels which are involved in motion of a part of an object that is imaged. As a consequence noise-breakthrough occurs in parts of an image involving motion, i.e. noise is displayed in parts involved in motion in a displayed image.